How to fix "warning: statement has no effect" in C++?

Question

Hello everyone, I'm writing code to find the middle of a link list but it returns me nothing when complied. Here is my code:

int numOfNodes(struct listNode * firstNode){
int countNodes = 0;
while(firstNode -> next != NULL){
countNodes ++;
}
return countNodes + 1;
}

int middleOfList(struct listNode * firstNode){
int count = 0;
int numb = numOfNodes(firstNode);
int previous;

while(count != numb / 2) {
if (count == (numb / 2) - 1){
previous = firstNode -> num;
count++;
** firstNode -> next;
}
else{
count++;
** firstNode -> next;
}
}
if(numb % 2 != 0) {
return firstNode -> next -> num;
}
else {
int total = ((firstNode -> next -> num) + previous) / 2 ;
return total;
}
}

The first function just counts the number of nodes there are in the link list. the second function however (middleOfList) gives me the warning where you see the ** in the function.(firstNode -> next). The complier is telling me that it has no effect but that is how I iterate through the link list to find the middle of it. This is probably the reason why it returns me 0 every time, meaning that it has no errors. Could someone please tell me where I am going wrong? All answers will be appreciated and thanks in advance.